1. FIELD OF THE INVENTION
The present invention relates to a focusing apparatus of a video camera or the like, and in particular to an electronically-controlled focusing apparatus which facilitates focusing using manual manipulation.
2. DESCRIPTION OF THE PRIOR ART
As shown in a schematic diagram of FIG. 1, a lens system of a video camera or the like principally comprises a focusing lens (front lens) group 17, a variator lens group 32, a compensator lens group 33, a diaphragm unit 31 and an imaging lens (master lens) group 34. The front lens group 17 functions to focus on each of desired objects to be photographed which are located at arbitrary distances. The variator lens group 32 provides a magnification changing function in accordance with zooming. The compensator lens group 33 moves with zooming and performs correction so as to prevent unsharp focusing during the zooming operation with respect to the object to be photographed. And the focusing lens group 34 functions to focus an image on an image pickup device 2. In a zoom lens used in such a video camera, focus is typically adjusted by movement of the front lens group. Focusing apparatuses are broadly divided into so-called auto focusing apparatuses in which auto focus adjustment is automatically performed and manual focusing apparatuses. In auto focusing apparatuses, an infrared distance measuring scheme or a TTL (Through The Lens) video scheme is used as described in National Technical Report, volume 31, No. 6, December 1985, pp. 65 to 67, for example. In general, the manual focusing apparatus interlocks with the front lens 17 and has such a structure that a distance ring 29 mounted on a lens barrel 30 is rotated and precisely adjusted directly by a hand to attain focusing.
On the other hand, demand for convenient to handle, inexpensive, handy and compact commodities has become stronger as the demand for a camera united with a VTR into one body such as a movie camera has rapidly expanded in recent years. As a commodity satisfying such demand, that as described in "Hitachi camera united with VTR, Mastax Movie C30", Hitachi, April 1987, p. 17 is announced. In this commodity, all lenses are incorporated within the movie casing for attaining compactness. Unlike the manual focusing apparatus, therefore, it is impossible to rotate the distance ring of a lens directly by a hand for focusing. Therefore, a so-called power focusing mechanism is adopted so that focusing may be performed by pressing a button switch when manual focusing is desired. By adopting this power focusing mechanism, both the compactness and convenience in handling are attained. Although a specific example of the power focusing mechanism is not described in the former prior art example, this is attained by circuit means as shown in FIG. 2, for example. In FIG. 2, A and B denote terminals used for applying auto focusing control signals. Numeral 7 denotes an automatic/manual changeover switch. SW.sub.1 and SW.sub.2 denote power focusing button switches. Only while buttons of SW.sub.1 and SW.sub.2 are pressed are their respective contacts connected. Numeral 10 denotes a focusing motor. Numeral 9 denotes a motor drive circuit comprising transistors Q1 to Q7 and resistors R1 to R6. Under the state that the automatic/manual changeover switch 7 is thrown to the manual side (i.e., the opened switch side) and the button switch 1 is pressed, the transistors Q1, Q2, Q3 and Q8 are in the off-state, and the transistors Q4, Q5, Q6 and Q7 are in the on-state. A motor current flows in a direction indicated by an arrow a through power supply (Vcc), the transistor Q4, the transistor Q7, and ground G to rotate the motor 10 (in a forward direction, for example). Under the state that the button switch 2 is pressed, the transistors Q4, Q5, Q6 and Q7 are in the off-state and the transistors Q1, Q2, Q3 and Q8 are in the on-state on the contrary. In this state, a motor current flows in a direction indicated by an arrow b through the power supply (Vcc), the transistor Q8, the motor 10, the transistor Q3 and the ground G. The rotation direction of the motor 10 is thus reversed. Under the state that the button switches are not pressed, the transistors Q1, Q2, Q3, Q5, Q6 and Q7 are in the on-state, and the transistors Q4 and Q8 are in the off-state. Since both ends of the motor are grounded respectively through the transistors Q3 and Q7, the motor current does not flow and the motor is in the stop state. In power focusing, the forward rotation, the backward rotation and stoppage of the motor are repeated and focusing is performed by fine adjustment, i.e., manual focusing using a motor is performed.
In a focusing apparatus having such a structure that the distance ring of a lens may be manipulated to rotate directly by a hand as described above, focusing can be performed rapidly and with high precision. However, it is difficult to make the commodity compact because of its structure, and it is somewhat inconvenient to handle. On the other hand, the power auto focusing apparatus is excellent in compactness of commodities. However, the power auto focusing apparatus has the disadvantages described below. In the case of power focusing, manipulation of button switches for repetitively causing the forward rotation, the backward rotation and stoppage of the focusing motor to perform focusing is simple and convenient. As a motor in case of power focusing, however, a DC motor is generally used. When it is attempted to move and stop the comparatively heavy front lens by using such a DC motor in accordance with button manipulation, it becomes difficult to stop the front lens at a desired lens position precisely and suitably because of influence of inertia and difference in sense from the case where the distance ring is directly rotated for focusing. If the rotation speed is lowered by decreasing the motor current, the influence of inertia can be reduced to some degree, resulting in focusing with comparatively high precision. However, the focusing time is disadvantageously prolonged.